This invention relates to the technology of measuring the water content of the soil, as well as a method of cultivation and an apparatus for cultivation that use the technology. The invention relates particularly to a method that measures the water content of the soil and performs pF conversion to determine its pF value in order to facilitate soil management and enable the saving of water, resources and labor, as well as a method and an apparatus for irrigation control which control the supply of water or nutrient (nutrient solution) to the soil on the basis of the measured pF value. The term xe2x80x9csoilxe2x80x9d as used herein refers to all materials that support the underground parts of plants such as root and subterranean stem; the term includes not only what is commonly called xe2x80x9csoilxe2x80x9d but also solid mediums such as sand particles, gravel stones, smoked coal and pumice.
In their cultivation on the soil, the growth of field crops is largely dependent on the soil-moisture content which can be controlled in various ways; in lands of high moisture content, enhanced drainage is performed whereas in lands of low moisture content, irrigation is performed, which is enhanced in dry seasons. In order to ensure a favorable growth of field crops, the accurate moisture content in the soil must be known.
High-grade vegetables such as corn salad and tomato are usually grown in crop fields but sometimes they need to be cultivated in open-area facilities and greenhouses with the environment being precisely controlled as in industrial plants. This cultivation method is called xe2x80x9cprotected cultivationxe2x80x9d. Cultivation in crop fields involves supplying fertilizers and other nutrient sources to the soil while applying water to the crop. In protected cultivation, solution culture is preferably adopted, according to which sand particles, gravel stones, smoked coal, etc. are laid down to make mediums which are supplied with aqueous nutrient solutions by irrigation. However, optimum irrigation has not always been achieved in the actual cultivation, particularly in solution culture.
Solution culture can be classified into three types, hydroponics, aeroponic and solid-medium culture. In solid-medium culture, continuous drip irrigation is commonly adopted. In continuous drip irrigation, timer or otherwise controlled automatic irrigation is the dominant approach but optimum irrigation is not always assured. This is because the amount in which the nutrient solution is absorbed by crops being cultured is dependent on various factors including the amount of solar radiation, as well as the temperature and humidity in the greenhouse. For example, the transpiration from crops being cultivated is very high if the amount of solar radiation is large and the greenhouse has high temperature and low humidity. On the other hand, the transpiration from crops being cultivated decreases on a rainy day. The absorption of nutrients by crops being cultivated is also largely dependent on the process of their growth and if they have grown up, the absorption of nutrients becomes very high. It is known that high-quality fruits with high sugar content can be obtained by reducing the water supply after they have grown to a certain extent. However, timer and otherwise controlled automatic irrigation is incapable of meeting those environmental conditions for cultivated crops at various stages of their growth unless the number of irrigations, the start time of irrigation and the duration of irrigation are daily set for new values.
This is not xe2x80x9ctimer or otherwise controlled automatic irrigationxe2x80x9d in the true sense of the term and it is highly doubtful whether optimum irrigation can really be achieved. For these reasons, timer or otherwise controlled automatic irrigation often involves over-irrigation in order to prevent wilting or other troubles of crops but then it has been impossible to avoid root rot due to over-irrigation and increased drainage (i.e., increased quantities of nutrient solution and water are discarded).
Speaking of the relationship between the moisture content of the soil and field crops, not all of the water in the soil is available to field crops and bound water in the soil is not available to the growth of field crops. The moisture content of the soil also varies with weather changes; the soil is filled with water if there is a heavy rain but thereafter the water is gradually absorbed by the lower soil layers and the moisture content of the soil decreases. The soil filled with water is equivalent to what occurs in hydroponics and its air permeability is too low to be suitable for open-field cultivation. If, at the subsequent stage, the moisture content of the soil decreases considerably to a level below a certain threshold, the root is no longer capable of sucking up water and the capillary network in the root is interrupted, causing the root to wither. Once this stage has been reached, the root will no longer recover from withering even if it is supplied with water; therefore it is necessary that the moisture content of the soil be kept higher than the lower limit defined by that stage.
Since the wetness of the soil is determined by the potential of water in the soil, it is held inappropriate that the wetness of the soil which is related to the cultivation of field crops should be simply expressed by the moisture content of the soil. A more preferred method is by expressing the wetness of the soil on the basis of its water potential.
One of the factors that describe the wetness of the soil is the xe2x80x9cpF valuexe2x80x9d. Being first proposed by R. K. Schofield in 1935, the pF value is an index for the matrix potential as a soil-water potential. The matrix potential is a drop in chemical potential resulting from the interactions between water and soil particles, as exemplified by capillary, intermolecular and Coulomb forces. Stated briefly, the matrix potential is the force by which water molecules are attracted to soil particles. The common logarithm of the absolute value of a matrix potential expressed by a graduation on a water column (cm) is called the pF value. The soil-water potential xcfx86 expressed by a graduation on a water column (cm) and the pF value are related by pF=log(xe2x88x9210.2 xcfx86).
The pF value is a quantity that describes the quality of water in the soil (which is a nutrient in solution culture). A near-zero pF value represents the state of the soil that is filled with water. The moisture that remains in the soil 24 hours after rainfall or irrigation is called field capacity and has a pF value of about 1.7; the water which is present in the range from the field capacity to the primary wiltig point (pF of 3.8) at which a crop starts to wither is called xe2x80x9cavailable waterxe2x80x9d. In practice, however, the growth of crops begins to experience troubles at a point where more water exists than at the primary wiltig point. The point is where the capillary network in a crop""s root is interrupted to stop the movement of water from the root. Called the rupture of capillary network, this point has a pF of about 2.7. Hence, for cultivation of crops, it is generally held that the pF value is suitably within the range from 1.7 to 2.7. For these reasons, the moisture present in the pF range of 1.7-2.7 is called xe2x80x9ceasily available waterxe2x80x9d and for cultivation of field crops in the soil, it is required to maintain this easily available water in the pF range of 1.7-2.7. The descriptions of the pF value and the soil-water potential may be found in xe2x80x9cDojo Kankyo Bunsekiho (Analyses of Soil Environment)xe2x80x9d, ed. by the Editors"" Committee on Analyses of Soil Environment under the supervision of the Society of Soil and Fertilizers of Japan, published by Hakuyusha, first printing in 1997, pp. 48-51; xe2x80x9cTsuchi no Kankyoken (Environment of Soil)xe2x80x9d, ed. under the supervision of Shingo Iwata, published by Fuji-Techno System, 1997, pp. 72-76; xe2x80x9cDojo Shindan no Hoho to Katsuyo (Soil Analysisxe2x80x94Methods and Applications)xe2x80x9d, Shunrokuro Fujiwara et al., published by Nosangyoson Bunka Kyokai, Corporation, 1996, pp. 72-77; and xe2x80x9cSaishin Dojogaku (Modern Soil Sicence)xe2x80x9d, ed. by Kazutake Kyuma, published by Asakura Shoten, 1997, pp. 101-107.
In the cultivation of field crops in the soil, irrigation and other operations are desirably performed on the basis of the pF value.
Among various methods for pF value measurement, tensiometry is known to be capable of direct field measurement on the soil.
A method of measurement that can be effectively used for controlling irrigation in actual cultivation must be capable of direct measurement of how much water can be held by the soil in a particular field. In ordinary fields, therefore, tensiometry is used as a simple method for measuring the pF value of the soil in the interest of management for optimum irrigation and the like. Tensiometry involves the use of an instrument called the tensiometer which consists of a porous ceramic cup (probe) and a rigid transparent poly(vinyl chloride) tube; the tensiometer is buried in the soil and filled with water so that the soil moisture has hydraulic continuity to the water inside the tube through the pores in the probe walls, whereupon the matrix potential of the soil equilibrates with the pressure inside the tube, making it possible to read the pressure inside the tube as the matrix potential of the soil. For details of tensiometry, see, for example, xe2x80x9cDojo Kankyo Bunsekihoxe2x80x9d, supra, pp. 59-62.
However, the conventional method of tensiometry requires in situ system of replenishment with water and management of the sensor (tensiometer) is quite cumbersome; it is therefore desired to ensure that the pF value of the soil can be measured by a simpler means or with a simpler system. Another difficulty is that depending on the quality of the soil to be measured, the use of tensiometry is sometimes unsuitable.
To be specific, tensiometry cannot be used with soil composed of coarse particles, for example, solid mediums made of coarse particles having porous surfaces such as pumice particles typically used in solution culture.
This is because the particles in the coarse medium do not make intimate contact with the entire surface of the probe and, hence, the water on such particles fail to have intimate contact with the probe surface, making it impossible to achieve correct measurement. Although it has been recognized that irrigation control on the basis of pF value as an index is also desirable in cultivation on solid mediums made of coarse particles, this need has never been met.
Currently, there is no alternative to tensiometry as a method capable of direct pF value measurement in the soil.
One of the methods that are drawing increasing attention today as means for investigating the water retentivity of the soil is by determining the volumetric soil water content from the measurement of its dielectric constant. Two practical approaches toward this goal are TDR (time-domain reflectometry) which determines the dielectric constant of the soil from the propagation time of electric pulses and FDR (frequency-domain reflectometry) which determines the dielectric constant of the soil from the frequency domain characteristics of electric pulses. In addition, ADR (amplitude-domain reflectometry) based on impedance measurement has been proposed as a more convenient and less costly method for measuring the volumetric soil water content. Details of these methods may be found in xe2x80x9cDojo Kankyo Bunsekihoxe2x80x9d, supra, pp. 62-64; Topp, G. C. et al. (1980), Electromagnetic determination of soil water content: Measurements in coaxial transmission lines, Water Resources Research, 16, 574-582; Haruhiko Horino and Toshisuke Maruyama (1993), TDR Measurement of Soil Moisture with Three-line Probe, Collected Papers of Japanese Society of Irrigation, Drainage and Reclamation Engineering, 168, 119-120; Kitahei Tatsumi et al. (1966), Field Measurement of Soil Moisture by FDR, Collected Papers of Japanese Society of Irrigation, Drainage and Reclamation Engineering, 182, 31-38; Makoto Nakajima et al. (1997), Soil Moisture Measurement by ADR, Proceedings of 1997 Spring Meeting of Japanese Association of Groundwater Hydrology, pp. 18-23. In particular, ADR permits very convenient measurements, assures high degree of correlation, can be performed with a simple and easy-to-maintain instrument for measurement, involves easy handling, and enables continuous measurement in a so-called xe2x80x9cmaintenance-freexe2x80x9d manner. However, the methods mentioned above are intended to determine the volumetric soil water content and are incapable of direct measurement of its pF value.
An object, therefore, of the present invention is to provide means for measuring the pF value of the soil by investigating its water retentivity without using tensiometry.
Another object of the invention is to provide a method in which the pF value of the soil is measured instantaneously and continuously through investigation of its water retentivity and the measured pF value is used to control irrigation. Still another object of the invention is to provide an apparatus for implementing the method.
Yet another object of the invention is to provide a method which provides means capable of measuring, particularly in a continuous manner, the pF value of the soil even if it cannot be measured directly by tensiometry or other methods and which controls irrigation on the basis of the measured pF value. A further object of the invention is to provide an apparatus for implementing this method.
A still further object of the invention is to provide a method for measurement of soil-water content which enables pF value based control of irrigation by using a simple and easy method for measuring the water retentivity of the soil which precludes the use of a tensiometer in solution culture on solid mediums, particularly in the case of porous, large-diameter particles such as pumice particles. Yet another object of the invention is to provide an apparatus for implementing this method. A further object of the invention is to provide a method which enables pF value based control of irrigation in cultivation on solid mediums. A still further object of the invention is to provide an apparatus for implementing the method.
The present inventors conducted intensive studies in order to attain the stated objects and noted that a correlation depending upon the type of the soil, namely, the soil texture, existed between the pF value and the volumetric soil water content that can be measured fairly easily by ADR and other conventional methods described above. It was found that by measuring the volumetric soil water content of interest after determining the correlation between volumetric water content and pF value for that soil, the pF value of the soil could be determined and used to control irrigation. The present invention has been accomplished on the basis of this finding.
Thus, the present invention provides a method for measuring the pF value (soil moisture tension) of the soil, comprising the steps of:
(a) preliminarily determining the correlation between pF value and volumetric water content of the soil to be measured;
(b) measuring the volumetric soil water content; and
(c) converting the value of the volumetric soil water content measured in the above step (b) to the corresponding pF value on the basis of the correlation between pF value and volumetric soil water content predetermined in the above step (a).